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Institut für Astronomie und Astrophysik

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Preprint B/02

Is RXJ1856.5-3754 a Quark Star?

Jeremy J. Drake (1), Herman L. Marshall (2), Stefan Dreizler (3), Peter E. Freeman (1), Antonella Fruscione (1), Michael Juda (1), Vinay Kashyap (1), Fabrizio Nicastro (1), Deron O. Pease (1), Bradford J. Wargelin (1), Klaus Werner (3)

(1) Smithsonian Astrophysical Observatory, MS-3, 60 Garden Street, Cambridge, MA 02138
(2) MIT Center for Space Research, Cambridge, MA 02139
(3) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany

To appear in: The Astrophysical Journal

Abstract. Deep Chandra LETG+HRC-S observations of the isolated neutron star candidate RXJ1856.5-3754 have been analysed to search for metallic and resonance cyclotron spectral features and for pulsation behaviour. As found from earlier observations, the X-ray spectrum is well-represented by a ~60 eV (7 × 105 K) blackbody. No unequivocal evidence of spectral line or edge features has been found, arguing against metal-dominated models. The data contain no evidence for pulsation and we place a 99% confidence upper limit of 2.7% on the unaccelerated pulse fraction over a wide frequency range from 10-4 to 100 Hz. We argue that the derived interstellar medium neutral hydrogen column density of 8 × 1019 <= NH <= 1.1 × 1020 cm-2 favours the larger distance from two recent HST parallax analyses, placing RXJ1856.5-3754 at ~140 pc instead of ~60 pc, and in the outskirts of the R CrA dark molecular cloud. That such a comparatively rare region of high ISM density is precisely where an isolated neutron star re-heated by accretion of interstellar matter would be expected is either entirely coincidental, or current theoretical arguments excluding this scenario for RXJ1856.5-3754 are premature. Taken at face value, the combined observational evidence - a lack of spectral and temporal features, and an implied radius Rinfinity=3.8-8.2 km that is too small for current neutron star models - points to a more compact object, such as allowed for quark matter equations of state.

Key words: stars: individual (RXJ1856.5-3754) - stars: neutron - X-rays: stars

Paper (63k gzip'ed Postscript including figures) (astro-ph/0204159)

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